Milk pumps are known, for example, from U.S. Pat. No. 4,813,932, on the one hand, and from EP-1 231 955, on the other hand. In both cases, a piston within the displacement space is moved by an actuation lever. In particular, working mothers pump mother milk in this way in order to be able to put it at the disposal of the baby during day time, while the child is under the care of a foster-mother or of a relative.
It is known that working mothers are subjected to considerable stress. Pumping mother milk takes a certain time which even intensifies the morning stress situation. Although manual pumps of the above described type are relative economical in price, many women do without them and often choose a more expensive motor pump. As a reason, often the quick fatigue of the hand is stated.
Such fatigue will particularly occur, if the pump is of a short-stroke type so that many pumping movements are necessary in order to achieve a sufficient suction effect. This applies in particular to designs having a relative short membrane, such as the rolling membrane, that seals a piston, according to WO 2004/058330 or according to EP-0 733 376 where the stroke is quite limited so that they have to be actuated more often to achieve the same result as, for example, a piston pump. Moreover, such membranes are quite sensitive to tension and are easily susceptible to fissures, because they are generally moved against their inherent elasticity.
Since overflowing of milk into the pumping space should be prevented, it has also become known (see WO 2006/000292) to insert a membrane over the whole suction cross-section, and to move this membrane indirectly by a piston and its suction effect. However, such design is quite space consuming and has, furthermore, the disadvantage that the piston motion has to be synchronous with the movement of the membrane; for example, if the membrane, when being inserted into the pump (e.g., after washing it after use), is inserted in a somewhat squeezed, and thus shortened way, whereas the piston executes its full length first stroke, an efficient and easy pumping work can no longer be performed.